Technical Field
The present disclosure relates to calibrating offsets in multiple membrane microphones and, in particular, offsets introduced in audio signals by channel amplifiers.
Description of the Related Art
Acoustic transducers, such as a microphone of a microelectromechanical system (MEMS), typically comprise a sensing structure for transducing acoustic pressure waves into an electrical signal (for example, by capacitive variation). The acoustic transducers may further comprise electronic circuitry for processing the electrical signals and supplying an output signal that may be analog or digital as in the case of a digital microphone. The output signal may be modulated in accordance with any type of modulation scheme such as a pulse-density modulation (PDM) modulation scheme. The output signal may then be made available to an external electronic system, such as for example a microcontroller of an electronic apparatus that incorporates the MEMS microphone.
FIG. 1 shows a schematic of a multi-membrane microphone 10. The multi-membrane microphone 10 (also referred to herein as a multiple membrane microphone) comprises a plurality of membranes 12, a charge pump 14, and an audio processing circuit 16. The audio processing circuit 16 comprises a plurality of amplifiers 18, a multiplexer 20, an analog-to-digital (A/D) converter 22 and a controller 24.
An input port 26 of each membrane 12 is electrically coupled to an output port 28 of the charge pump 14. Further, an output port 30 of each membrane 12 is electrically coupled to an input port 32 of a respective amplifier 18 of the plurality of amplifiers 18. An output port 34 of each amplifier 18 of the plurality of amplifiers 18 is electrically coupled to a respective input port 36 of a plurality of input ports 36 of the multiplexer 20. Further, a selection signal input port 38 of the multiplexer 20 is electrically coupled to a selection signal output port 40 of the controller 24.
As shown in FIG. 1, an output port 42 of the multiplexer 20 is electrically coupled to an input port 44 of the A/D converter 22, and an output port 46 of the A/D converter 22 is electrically coupled to an input port 48 of the controller 24 at output node 50. An output signal of the audio processing circuit 16 may be sensed at the output node 50.
The input port 26 of each membrane 12 receives a charging signal 52 from the output port 28 of the charge pump 14, whereby the charging signal 52 is used to maintain a constant charge on the sensing capacitor of the membrane 12. The constant charge is maintained to enable the membrane 12 to better transduce an impinging sound pressure signal 54. As a result of the sound pressure signal 54 impinging on the membrane 12, the membrane 12 produces a respective sensing signal 56 at its output port 30.
The membranes 12 may have different sensitivities and may have different signal-to-noise ratio (SNR) ratings. The controller 24 of the audio processing circuit 16 may switch between membranes 12 depending on a level of the sound pressure signal 54. For example, one membrane 12 may have greater sensitivity than another membrane 12 and may be used when the sound pressure signal 54 impinging on the membranes 12 is below a threshold, whereas the other membrane having the lower sensitivity may be utilized when the sound pressure signal 54 is greater than the threshold. Switching between audio detection by the plurality of membranes 12 improves the audio detection capability of the microphone 10.
The plurality of amplifiers 20 receive a respective sensing signal 56 at their respective input ports 32. Each amplifier 20 of the plurality of amplifiers 20 amplifies a signal level of the respective sensing signal 56 and outputs an amplified signal 58 at the respective output port 34 of the amplifier 18. The multiplexer 24 receives the plurality of amplified signals 58 at the respective plurality of input ports 36. The multiplexer 20 also receives a selection signal 60 from the controller 24 at its selection signal port 38. The selection signal 60 indicates selection of one of the plurality of amplified signals 58 for processing. Based on the selection signal 60, the multiplexer 20 outputs, at its output port 42, one of the amplified signals 58 as a selected amplified signal 62.
The selection signal 60 indicates an audio channel selected for processing by the controller 24, whereby the audio channel comprises the membrane 12 and amplifier 18 corresponding to the selected amplified signal 62. The A/D converter 22 receives the selected amplified signal 62 at its input port 44 and converts the selected amplified signal 62 from analog format to digital format. The A/D converter 22 outputs, at its output port 46, an output signal 64 that is the result of the conversion.
The output signal 64 is received by the controller 24 at its input port 48. The controller 24 uses feedback control to control audio channel selection. For example, based at least in part on a signal power level or noise power level of the output signal 64, the controller may determine to switch between audio channels. For example, if the signal power level of the sound pressure signal 54 falls below a threshold, the controller 24 may select the membrane 12 having a higher sensitivity for detecting the sound pressure signal 54. Conversely, the controller 24 may select the membrane 12 having a lower sensitivity for detecting the sound pressure signal 54. The controller 24 selects the membrane 12 for detecting the sound pressure signal 54 by outputting the selection signal 60 indicating the selection of the amplified sensing signal 58 corresponding to the membrane 12.